This invention relates to the field of communication electronics, and more particularly, to a method of performing antenna diversity in a spread spectrum wireless local area network using a spread spectrum transceiver.
Wireless or radio communication between separated electronic devices is widely used. For example, a wireless local area network (WLAN) is a flexible data communication system that may be an extension to, or an alternative for, a wired LAN within a building or campus. A WLAN uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. Accordingly, a WLAN combines data connectivity with user mobility, and, through simplified configurations, also permits a movable LAN.
Over the past several years, WLANs have gained acceptance among a number users including, for example, health-care, retail, manufacturing, warehousing, and academic areas. These groups have benefitted from the productivity gains of using hand-held terminals and notebook computers, for example, to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized and used as a general purpose connectivity alternative for an even broader range of users. In addition, a WLAN provides installation flexibility and permits a computer network to be used in situations where wireline technology is not practical.
In a typical WLAN, an access point is provided by a transceiver, that is, a combination transmitter and receiver, connects to the wired network from a fixed location. Accordingly, the access transceiver receives, buffers, and transmits data between the WLAN and the wired network. A single access transceiver can support a small group of collocated users within a range of less than about one hundred to several hundred feet. The end users connect to the WLAN through transceivers which are typically implemented as PC cards in a notebook computer, or ISA or PCI cards for desktop computers. Of course the transceiver may be integrated with any device, such as a hand-held computer.
Spread spectrum communications have been used for various applications, such as cellular telephone communications, to provide robustness to jamming, good interference and multi-path rejection, and inherently secure communications from eavesdroppers, as described, for example, in U.S. Pat. No. 5,515,396 to Dalekotzin. The patent discloses a code division multiple access (CDMA) cellular communication system using four Walsh spreading codes to allow transmission of a higher information rate without a substantial duplication of transmitter hardware. U.S. Pat. No. 5,535,239 to Padovani et al., U.S. Pat. No. 5,416,797 to Gilhousen et al., U.S. Pat. No. 5,309,474 to Gilhousen-et al., and U.S. Patent No. 5,103,459 to Gilhousen et al. also disclose a CDMA spread spectrum cellular telephone communications system using Walsh function spreading codes. The disclosures of each of these patents is hereby incorporated by reference in their entirety.
Antenna diversity is well known in communications where two or more antennas are switched when the same transmission is received at two different locations.
Examples of antenna diversity systems and method are disclosed in U.S. Pat. Nos. 5,216,434; 5,369,801; 5,530,926; and 5,748,676. A desired antenna is selected. The expectation is that the received signals will take two different paths, such as through bouncing off walls and through doors, and thus, one antenna will have and receive the better signal for demodulation and even transmission. However, many prior art techniques have taken the approach that the stronger signal is the better signal. However, often a weaker signal is the less corrupted signal and is advantageous for demodulation.
It is therefore an object of the present invention to provide a method of performing antenna diversity in spread spectrum transceiver and related systems that allows a better metric for selecting an antenna.
In accordance with the present invention, a method of performing antenna diversity in a wireless spread spectrum communication system comprises the steps of receiving on each of two respective spaced antenna of a spread spectrum receiver a spread spectrum phase shift keyed (PSK) package signal having data symbols formed from high rate mode chips. The method also comprises determining the bit sync peak sample within the packet signal for each antenna and subtracting the predetermined bit sync samples a predetermined number of chips on either side of the bit to sync peak sample from the peak for each antenna. The method also comprises selecting the antenna having the higher value obtained in the subtracting step accomplished for each antenna.
In accordance with another aspect of the present invention, the packet signal includes a header having a first antenna diversity period and a second antenna diversity period between the header and the start of high rate data in the packet signal for redoing antenna diversity. Time intervals can also be added within the second antenna diversity period for redoing antenna diversity including a code set distance time interval (such as a Walsh distance) for a currently selected antenna; a guard time interval for antenna switching; a timing and carrier phase acquisition time interval for the other unselected antenna; a code set distance time interval for the unselected antenna; a guard time interval for antenna switching and decay time; and a timing and carrier phase acquisition time interval for the selected antenna.
The high rate M-ary code set portion (such as a Walsh) can be inserted within the second antenna diversity period. The packet signal can also comprise code vectors where a plurality of code set vectors are received onto each antenna. For example, eight Walsh basis vectors could be received on each antenna. The code set distance can also be measured to determine the error in demodulated symbols on the selected antenna. The code set distance can be measured by determining the minimum correlation of code set vectors transmitted sequentially on one of either the I or Q channel. The maximum of the absolute value of the cross correlation of the code set vectors on both I and Q channels can be determined without the correlation peaks of the determined minimum correlation. The result of the absolute value is subtracted from the determined minimum correlation value to obtain the code set distance. When Walsh basis vectors are used, 8 Walsh vectors can be used.
The spread spectrum transceiver that performs antenna diversity in a wireless spread spectrum communication system can comprise two respective spaced antenna that each receive a spread spectrum phase shift key packet signal having data symbols formed from high rate mode chips. The transceiver also includes means for determining the bit sync peak sample within the packet symbol for each antenna. The transceiver also includes means for subtracting predetermined bit sync samples a predetermined number chips on either side of the bit sync peak from the peak for each antenna and means for selecting the antenna having the higher value obtained in the subtracting step. The method and associated circuitry-can use the taps out of the correlator as known to those skilled in the art. For example, there could be a tap of 22 samples in each of the sample intervals.
A high rate data frame for use in a wireless spread spectrum communication used in a wide area local area network (WLAN) is also disclosed and includes a preamble portion and a header portion and a code set high rate portion used for antenna diversity of two antenna and positioned after the head portion. The code set high rate portion includes a code set distance time interval for a currently selected antenna; a guard time interval for antenna switching; a timing and carrier phase acquisition time interval for the other unselected antenna; a code set distance time interval for the unselected antenna; a guard time interval for antenna switching and decay time; and a timing and carrier phase acquisition time interval for the selected antenna.